Gtkprintunixdialog: file picker has no access to files in flatpak
Flatpaked apps has to enable at least --filesystem=home in order to give the gtkprintunixdialog access to files when picking a file, for to file printing. I think the problem is that the dialog doesn't use GtkFileChooserNative widget.
https://gitlab.gnome.org/GNOME/gtk/blob/master/gtk/gtkprinteroptionwidget.c#L542
Related issue: https://github.com/fabiocolacio/Marker/issues/201